(Adapted from investigator's abstract): Positron Emission Tomography (PET) is a noninvasive medical imaging technique which provides an image of a positron emitting radio pharmaceutical and its distribution in the body on a metabolic level. It is becoming an important fundamental tool for both clinical diagnosis and medical research. The heart of this instrument is a large, circular array of gamma ray detectors. The complexity, cost and difficulty of building an instrument with hundreds of these bulky and fragile photosensors is a significant problem. The availability of a suitable solid state sensor to replace the phototubes and scintillator crystals would be a major breakthrough in the design of high resolution positron tomography cameras. Such a sensor would detect the 511 keV photons directly to produce an electronic pulse without an intermediate optical conversion. It would be a small, stable device which would make construction of rugged, compact multi-detector arrays for PET systems possible. At Radiation Monitoring Devices, Inc., gamma ray detectors made from a new semi-conductor material, thallium bromide (T1Br) have been investigated. T1Br has the same effective Z as bismuth germinate (BGO), the leading scintillator material used for PET, and higher density. Preliminary results indicate T1Br has potential for low noise and high energy resolution. The properties of this material make it a unique semiconductor which appears to have the characteristics required to finally make a semiconductor based PET camera.